Video: Introduction to Recrystallization

Purifying Compounds by Recrystallization is a video description that provides background and a good illustration of how to perform a recrystallization in the lab. Note: If you are off campus, you must use a VPN connection to the UR network in order to access this video.

General Protocol

In general, a recrystallization should be performed in an Erlenmeyer flask using a minimum amount of near-boiling solvent. You want the final solution to be saturated with your desired compound near the solvent’s boiling point. When the solution cools, the compound should exceed its solubility in the solvent and crystallize out of solution. The crystallization step should involve very slow cooling of the solution to room temperature. If crystal formation does not begin after the flask has cooled, you may induce formation by scratching the bottom of the flask with a glass rod or by seeding the solution with a crystal of pure product. You also may wish to cool the flask in an ice-water bath after the solution has reached room temperature in order to increase the yield of crystals.  In the final step, the crystals are collected by vacuum filtration and washed with a small volume of cold solvent in order to remove any solution or contaminant adhering to the crystal surface. The crystals are then dried before further analysis. 

1. Choosing the Solvent

Remember: “Like dissolves like” Choose a solvent with polarity that does NOT match your solute!

a. Place a few crystals of the solid in a test tube.

b. Add a few drops of the prospective solvent. If the solid dissolves, the solvent is a poor choice. If the solid does not dissolve, proceed to step c.

c. Warm the mixture in a hot water or sand bath. If the crystals do not dissolve, add a few more drops of solvent. If the crystals still do not dissolve, the solvent is a poor choice. If the crystals dissolve upon heating, proceed to step c.

d. Remove the test tube from the heat and allow it to cool. If crystals form, the solvent is a good choice. If not, a different solvent must be selected.

2. Dissolving the Solid

a. Use a hot plate to preheat the solvent to near its boiling point. Note: Always keep a boiling stick, boiling stone, or stirrer in the solvent to prevent it from “bumping” (flash boiling out of the flask).

b. Dissolve the crude solid in a minimum amount of hot solvent: Add small amounts (1-2 mL) of solvent at a time. Swirl the mixture and keep adding small quantities of hot solvent until the solid dissolves. If the mixture cools, you may heat it on the hot plate, but do not heat the solid by itself or with too little solvent. Doing so may cause the solid to melt and form an oil in the solvent rather than dissolving in it. Note: Always have a stir rod in the heated mixture to facilitate mixing and prevent bumping!

3. Removing Insoluble Impurities (only if necessary)

a. Perform a vacuum filtration on the hot mixture to remove insoluble impurities.

b. The filtrate should be reheated to dissolved any solids that precipitated during the filtration.

4. Crystallization

a. Allow the hot solution of dissolved solute to cool to room temperature slowly and undisturbed.

b. To prevent rapid cooling, you may set the flask on a surface that does not easily conduct heat, such as a hot pad or a stack of paper towels.

c. If crystallization does not begin on its own, scratch the bottom of the flask with a glass rod or add a seed crystal of the solid. Note: If crystallization still does not occur, you may have added too much solvent. Heat the solution and allow some of the excess solvent to evaporate.

d. After crystals have appeared and the solution has reached room temperature, cool the flask in an ice-water bath to increase the yield of crystals.

5. Collection and Drying

a. Use a Buchner or Hirsch funnel to collect the crystals by vacuum filtration.

b. Rinse the crystals with a small portion of ice-cold solvent.

c. Allow air to pass over the crystals until they are relatively dry. For the most accurate analysis, allow the crystals to air dry for at least 24 hours or remove residual solvent by mild heating and/or exposure to vacuum.

Video: Theory of Recrystallization

This background video provides an easy to understand description of the theory of recrystallization.